Changing flavors

Pooling resources:
A pool holding detectors at the Daya Bay neutrino experiment fills with ultrapure water. In early March, scientists recorded an important value related to neutrino behavior. The Daya Bay experiment is a multinational consortium that includes a team from William and Mary.

Physicists find key value of neutrino oscillation

by Joseph McClain
| March 8, 2012

An international team of physicists has reported the first
set of observations detailing important behavior of neutrino oscillation, an
accomplishment that is a necessary step to additional experiments intended to
answer fundamental questions about the makeup of the universe.

Physicists operating the Daya Bay neutrino experiment in
China have observed a neutrino “mixing angle” known as θ13 (pronounced “theta
one-three.”) Robert McKeown says Daya Bay’s discovery of θ13 is the final piece
needed to complete the understanding of how these mysterious particles
oscillate between three forms, or “flavors,” to another. Neutrinos are
byproducts of nuclear reactions, emitted in enormous quantities by the sun, by
nuclear activity in the core of the earth and by nuclear power plants. Despite
their large numbers, neutrinos are extremely hard to detect.

McKeown, the Governor's Distinguished CEBAF Professor in
William & Mary’s physics
department as well as deputy director for science at Jefferson Lab, leads the William
& Mary contingent working on the Daya Bay experiment. McKeown and his collaborators
built a set of instruments designed to observe the stream of particles emitted
from a nuclear power facility.

“Neutrino physics is a very important part of particle
physics these days,“ McKeown said. “The first thing that knowing this particular
mixing angle allows us to do is to confidently mount the next series of
experiments that are to be done.”

He said the Daya Bay results give physicists the scientific
grounds to proceed with more ambitious neutrino experiments. McKeown has been
involved in planning of the Long-Baseline Neutrino Experiment (LBNE), which,
when constructed, will send a neutrino beam more than 1,000 kilometers through
the earth from Fermilab, in Batavia, Ill., to a detector that will be located
outside Lead, S.D.

The LBNE (which has yet to be funded, let alone constructed)
and similar experiments seek to use neutrinos to observe a phenomenon known as
CP symmetry violation.

“Basically it means that particle reactions and antiparticle
reactions are different,” McKeown said. “This is a very important feature that
would allow physicists to explain how the universe seems to contain matter, but
not antimatter.”

McKeown’s William & Mary group has two important
functions at Daya Bay. First, the team had
substantial responsibility for the installation, commissioning and calibration
of the detection apparatus.

“Since this
experiment is so precise, the equipment needs to be calibrated very, very
precisely,” McKeown said, adding that Daya Bay has eight detectors and each
detector has three units. “Not only are the detectors deep underground,
but they’re also underwater. It’s a complicated set of equipment and a
complicated set of data that needs to be analyzed.”

The William & Mary team also kept account of the
neutrinos detected by the apparatus. Wei Wang, William & Mary research
scientist, spent much of his time at Daya Bay site overseeing the installation
and calibration of the detector equipment, but McKeown said much of the team’s
work can be done here in the United States, working with data sent from the
detectors and the reactor plant.

China and the United States lead the Daya Bay Reactor
Neutrino Experiment, which includes participants from Russia, the Czech
Republic, Hong Kong, and Taiwan. The Chinese effort is led by Yifang Wang of
the Institute of High Energy Physics, while leaders of the U.S. effort, in addition to McKeown, include
project manager Bill Edwards of Lawrence Berkeley National Laboratory, Steve
Kettell of Brookhaven National Laboratory and Karsten Heeger of the University
of Wisconsin.

McKeown was interviewed about the discovery for
a story in the top journal Science.